The capital cost of drilling and evaluating a well, such as an oil or natural gas well, is extremely high, and for this reason, any time a well is not producing, including time spent evaluating the formation, involves considerable expense. Thus, there exists an on-going desire in the exploration and production industry for tools and techniques that gather as much information about a formation with as little interruption between drilling and production as possible.
A variety of well logging techniques has been developed to evaluate subsurface earth formations surrounding a borehole. A number of such techniques involve emitting neutrons into the formation and evaluating the results of neutron interactions with formation nuclei. For example, the hydrogen index and porosity of the formation surrounding a borehole may be investigated using neutron porosity logging. Neutron porosity logging takes advantage of the facts that hydrogen strongly moderates fast neutrons and that the pore spaces of earth formations tend to be filled with hydrogen rich fluids, such as hydrocarbons and water. In some types of neutron porosity logging, the borehole and surrounding formation are irradiated with neutrons emitted from a neutron source housed within a logging tool, and populations of thermal and/or epithermal neutrons from the borehole and formation are detected and counted at one or more locations away from the neutron source. The detected counts or count ratios are correlatable with hydrogen index and porosity. In another form of neutron porosity logging, the borehole and surrounding formation are irradiated with discrete bursts of neutrons from a pulsed neutron source, and the time rate of decay, or die away, of epithermal neutron counts at one or more locations away from the neutron source is determined in addition to count rate ratios. These and other neutron logging methods and tools are described, for example, in U.S. Pat. No. 3,483,376 to Locke et al., U.S. Pat. No. 4,423,323 to Ellis et al., U.S. Pat. No. 4,760,252 to Albats et al., U.S. Pat. No. 5,051,581 to Hertzog et al., U.S. Pat. No. 5,349,184 to Wraight, and U.S. Pat. No. 5,789,752 to Mickael.
The macroscopic thermal neutron capture cross-section, commonly referred to as sigma, may also be determined using neutron logging techniques. The borehole and surrounding formation are irradiated with neutrons, and the various interactions of neutrons with constituent nuclei cause the energy of the neutrons to decrease. At thermal energy levels, the neutrons may be captured, or absorbed, by various nuclei, which cause the nuclei to emit gamma rays. The thermal neutron capture cross section may be determined from monitoring the decay of the thermal neutron and/or the gamma ray population, and provides information that may help, for example, to distinguish salt water from hydrocarbon and to indicate the presence of shale in the formation. Sigma measurements and thermal decay logging methods and tools are described, for example, in U.S. Pat. No. 4,721,853 to Wraight and U.S. Pat. No. 5,235,185 to Albats, et al.
Another type of logging technique that utilizes neutrons is gamma ray logging. When a formation is irradiated with high-energy neutrons, the neutrons can interact with certain nuclei in the formation to produce gamma rays via either inelastic neutron scattering or neutron capture. Neutron capture has been described above. In inelastic neutron scattering, a high-energy neutron collides with and excites a nucleus, causing the nucleus to promptly emit gamma rays. Gamma rays produced from neutron capture are called capture gamma rays, and gamma rays produced through inelastic scattering are called inelastic gamma rays. The emitted gamma ray energies are measured and analyzed to estimate the abundances of certain elements in the formation, such as silicon, calcium, chlorine, hydrogen, sulfur, iron, titanium and gadolinium. Various gamma ray logging techniques and tools are described, for example, in U.S. Pat. No. 4,390,783 to Grau, U.S. Pat. No. 4,507,554 to Hertzog et al., U.S. Pat. No. 5,021,653 to Roscoe et al., U.S. Pat. No. 5,081,351 to Roscoe et al., U.S. Pat. No. 5,097,123 to Grau et al., U.S. Pat. No. 5,237,594 to Carroll, and U.S. Pat. No. 5,521,378 to Roscoe et al.
Each of the patents mentioned above is incorporated herein by reference in its entirety.